Yes, computers have been using time-slices to multitask on single core processors since before the Intel 486. - Assuming you are running a multitasking multi-user OS (such as Windows). - People did it back then, and the systems of today are thousands of times more powerful.
Even if your system did not meet the requirements to run Windows, so long as it actually loads, then you will be able to do this - There might be a slight performance impact, if you have 2GB of more of RAM you may not even notice the time difference in either download and scan times with a modern SATA (AHCI, NCQ) HDD.
Yes, you can do this. However this will divide the speed of your processor between two tasks making the download or scan slower.
It also has the tendency to freeze your PC causing you to reboot the unit and start all over again.
Take note since youre doing a scan and a download, the new download will not be included in the scan.